The present invention relates to a ball screw mechanism preferable for use in an extreme environment such as space, vacuum environment, or the like. More specifically, the present invention relates to a ball screw mechanism using a solid lubrication film with high precision.
As is known, space is a thermal vacuum environment and it is difficult to make fluid lubrication with use of oil or grease. Therefore, ball screw mechanisms forming part of various drive mechanisms and the like assembled in an aerospace vehicle adopt a solid lubrication system using a solid lubricant such as molybdenum disulfide or the like.
A known solid lubrication system is a method has in which solid lubrication films are formed on a ball, a screw shaft, and a thread groove of a nut by sputtering, plating, or coating.
In this kind of ball screw mechanism, a conduit member called a bypass member having a ball circulation path for sliding guide of balls is assembled and provided between both end portions of a thread groove of a nut and a bypass circuit is thereby formed so as to communicate the both end portions of the thread groove. As a result, when the screw shaft is driven to rotate, the screw shaft is driven to rotate in relation to the nut by an action of balls. The balls interposed between the thread groove of the screw shaft and the nut are circulated and moved through the ball circulation path of the conduit member, spiral engagement between the screw shaft and the nut is adjusted so that the nut is guided in an axial direction.
However, in a ball screw mechanism of the solid lubrication system described above, it is difficult to expect a damping effect from a solid lubricant inserted between members which are in contact with each other. Consequently, vibration or noise may be incurred when balls circulate if positional dislocation occurs at a gap between a thread groove of a nut, a ball circulation path of a conduit member, and a thread groove of a screw shaft. Thus, a ball screw mechanism or the like of a solid lubrication system generally has a problem of difficulties in drive control with high precision.
Also as described above, if the circulation path of balls includes positional dislocation of a predetermined value or more, fatigue and abrasion proceed and lead to shortened lifetime because of vibration and impact caused when balls circulate. As a result, it is difficult to satisfy requirements for use in a space environment which demands for long lifetime.
In order to solve problems described above, it may be considered that an above-mentioned ball screw mechanism used, for example, in space should be processed and manufactured with high precision which will hardly incur positional displacement of gaps or the like of a nut, conduit member, and screw so that vibration and noise can be reduced.
However, since there are boundaries of improvements in processing precision, a problem lies in that the manufacturing and processing of such a mechanism is very complicated. Further, even if a nut or a conduit member can be processed with high precision, the precision of the entire ball screw mechanism may be restricted by the precision of a solid lubrication film coated on the surfaces of the nut and conduit member. Specifically, at present, there are boundaries of improvements in precision of a thickness and the like of a solid lubrication film coated on those members. Therefore, precision of dimensions of those members and dimensions relative to balls are restricted by precision of a thickness and the like of the solid lubrication film, and this restriction limits precision of the entire ball screw mechanism.
In addition, the thickness of a solid lubrication film may change or a part thereof may be broken due to abrasion during use. In this case, a clearance between members in contact with each other is greatly changed. The precision of a ball screw mechanism is thus limited by a time-based factor.
The above-described situation is not limited to a space environment but is common to an extreme environment such as a vacuum environment on earth. In general, a ball screw mechanism using a conventional solid lubrication film are subjected to restrictions concerning improvements in precision as described above and attains only short lifetime. It is therefore difficult for such a ball screw mechanism to satisfy specification requirements from an extreme environment such as a space environment or the like where maintenance services or inspection services are difficult.